Problem: A circular garden is enlarged so that the new diameter is twice the old diameter. What is the ratio of the original area to the enlarged area? Express your answer as a common fraction.
Solution: If any linear dimension (such as radius, side length, height, etc.) of a two-dimensional figure is multiplied by $k$ while the shape of the figure remains the same, the area of the figure is multiplied by $k^2$.  Since the new diameter is 2 times the original diameter, the new area is $2^2=4$ times the old area.  Therefore, the ratio of original area to new area is $\boxed{\frac{1}{4}}$.